This invention relates to a fuel tank inerting system, and more particularly to a process and device for controlling oxygen content within a fuel tank.
An aircraft fuel tank includes a liquid fuel filled portion and a fuel vapor filled portion known as the fuel tank ullage. Oxygen mixed with the fuel vapor within the ullage creates a potentially hazardous condition. Systems that replace oxygen-containing air with an inert gas substantially reduce the risks of this potentially hazardous condition. Some systems utilize tanks of nitrogen rich air for replacing the oxygen containing air within the fuel tank ullage. An on-board inert gas generating system that replaces oxygen rich air with nitrogen-enriched air is currently under development for use in aircraft.
On-board inert gas generating systems utilize an air separation module that removes oxygen to produce a nitrogen-enriched air stream that fills the fuel tank ullage. Typically the fuel tank is vented to the ambient environment and experiences changes in pressure responsive to changes in altitude. Changes in ambient pressure affect the performance of the air separation module and the rate of flow of nitrogen-enriched air into the fuel tank ullage. Such changes in the rate of flow of nitrogen-enriched air result in a change in the amount of oxygen within the fuel tank.
Current systems utilize fixed flow orifices to control nitrogen-enriched airflow through the air separation module. A high flow orifice is utilized at high pressures, and lower altitudes, and a low flow orifice is utilized during all other conditions. The fixed orifices used to control flow of nitrogen-enriched air are not adaptable and cannot provide a variable flow rate in response to ambient conditions. Disadvantageously, the use of fixed orifices can result in more oxygen than desired within the fuel tank under certain conditions.
Further, the use of fixed orifices require a greater capacity air separation module to accommodate higher pressures at lower altitudes. It is therefore desirable to develop an on-board inert gas generating system that is adaptable to differing environmental conditions to minimize oxygen content within fuel tank ullage.